Method, apparatus and system, providing a one-time programmable memory device

ABSTRACT

Disclosed are apparatus, system and methods of programming and readout of a one-time programmable memory device having an array of memory cells, where the cells include an anti-fuse element and an in-cell amplifier transistor. Circuitry configured for programming and correlated double sampling readout of the cells is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/600,202, filed Nov. 16, 2006 now U.S. Pat. No. 7,593,248, theentirety of which is incorporated herein by reference.

FIELD OF THE INVENTION

Disclosed embodiments relate to a method, apparatus and system providinga one-time programmable memory device.

BACKGROUND OF THE INVENTION

Memory can generally be characterized as either volatile ornon-volatile. Volatile memory, for example, random access memory (RAM),requires constant power to maintain stored information. Non-volatilememory does not require power to maintain stored information. Variousnon-volatile memories include read only memories (ROMs), erasableprogrammable read only memories (EPROMs), and electrically erasableprogrammable read only memories (EEPROMs).

A one-time programmable memory device is a type of ROM that may beprogrammed one time and may not be reprogrammed. Some one timeprogrammable memory devices use an anti-fuse as the programmableelement. An anti-fuse may exist in one of two states. In its initialstate, (“un-programmed”) the anti-fuse functions as an open circuit,preventing conduction of current through the anti-fuse. Upon applicationof a high voltage or current, the anti-fuse is converted to a secondstate (“programmed”) in which the anti-fuse functions as a line ofconnection permitting conduction of a current. During a readout of thedevice, an un-programmed anti-fuse corresponds to a logic value, forexample “0”, and a programmed anti-fuse represents another logic value,for example “1”.

An anti-fuse may be implemented using a capacitor or a MOSFET. Whenprogramming an anti-fuse MOSFET, the process begins with application ofvoltage stress to the MOSFET gate, which causes defects to appear in thegate-oxide. As the defect density increases, eventually a critical levelis reached where a current may flow through the oxide through a chain ofdefects. The thermal effects of the current solidifies this newly formedconductive channel, or “pinhole,” through the oxide. For a capacitoranti-fuse, a programming voltage causes a breakdown in the capacitordielectric and a resulting short across the capacitor electrodes in asimilar manner as described above.

FIG. 1 shows a schematic of one proposal for anti-fuse cells arranged inan array 10 of a one-time programmable memory device 10. Each cell 20contains an anti-fuse element 30, common gate transistor 40 connected toone node of anti-fuse element 30, and switch transistor 50 having a gateconnected to a word line WL1 and one source/drain terminal connected toa bit line BL1 and another connected to transistor 40. A second node ofthe anti-fuse element 30 is connected to a programming voltage sourceline V_(CMN).

During the readout, the anti-fuse element 30 is directly driving theentire column output line, shown as bit line BL1, which has a high load.In order to meet typical programming and readout speed requirements, alow resistance anti-fuse is required to drive the high column load,which in turn requires a large common gate transistor 40 and switchtransistor 50 to deliver the current necessary to lower the resistanceof the anti-fuse.

A method, apparatus and system providing an improved anti-fuse cell andarray structure are desired which provide a smaller anti-fuse area,increased readout speed, and lower power requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of a proposed anti-fuse memorycell array;

FIG. 2 is a schematic circuit diagram of an embodiment of an anti-fusememory cell array;

FIG. 2A is a schematic circuit diagram of an embodiment of an anti-fusememory array including a comparator for a readout circuit.

FIG. 2B is a schematic circuit diagram of an embodiment of an anti-fusememory array including a sense amplifier for a readout circuit.

FIG. 2C is a schematic circuit diagram of an embodiment of an anti-fusememory array including readout circuitry for correlated double sampling.

FIG. 2D is a schematic circuit diagram of an embodiment of an anti-fusememory array including a readout amplifier for a readout circuit.

FIG. 3 is a semiconductor side view of an anti-fuse MOSFET and commongate transistor.

FIG. 4 is an embodiment of an three-transistor anti-fuse memory cellhaving an anti-fuse with a thinned gate oxide.

FIG. 5 is a timing diagram for programming the embodiments of FIG. 2.

FIG. 6 is a timing diagram for readout of the embodiment of FIG. 2C.

FIG. 7 is a block diagram of a processor system, e.g. a digital camera,incorporating an embodiment of an anti-fuse memory cell memory device.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and illustrate specificembodiments in which the invention may be practiced. In the drawings,like reference numerals describe substantially similar componentsthroughout the several views. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized, and that structural, logical and electrical changes may bemade within the bounds covered by the disclosure.

Referring now to the drawings, FIG. 2 illustrates an embodiment of aone-time programmable memory device having an array 60 of anti-fusememory cells 70. The array 60 of anti-fuse memory cells 70 (“anti-fusecells”) are accessible by row driver and decoder 140 and column decoder160. The anti-fuse cell 70 includes an anti-fuse element 80, a commongate transistor 90, a switch transistor 100, a row-select transistor110, an amplifier transistor 120, and a storage region 130, e.g., afloating diffusion region. The anti-fuse cell 70 is accessible forprogramming and readout via signals on bit line BL1, word line WL1, andword read line WR1. Word line WL1 and word read WR1 are connected to therow driver 140 output and are shared by an entire row of anti-fuse cells70. Bit line BL1 is controlled by the column decoder 160 and is sharedby an entire column of anti-fuse cells 70. The gates of common gatetransistor 90 of all cells are commonly coupled to a voltage source lineproviding a common gate voltage V_(CMN).

Vpp is a programming voltage applied to line 135 which is coupled to oneor more of each of the anti-fuse elements 80. The voltage Vpp is highenough to program anti-fuse elements 80 when applied. Vdd is theanti-fuse cell 70 power supply voltage and is high enough such that whenVdd is applied to a bit line, e.g. BL1 through transistor 100 or 90 andto anti-fuse element 80, Vpp−Vdd results in a voltage potentialinsufficient to program anti-fuse element 80. FIG. 2 shows fouranti-fuse cells 70, 170, 180 and 190 arranged in two rows and twocolumns, however it should be understood that the array could comprisehundreds or thousands of cells or more arranged in a plurality of rowsand columns. Column readout circuitry 150 will be described furtherbelow.

Anti-fuse element 80 could be implemented in a number of ways known inthe art, for example, using a capacitor or a MOSFET. Not limited to thefollowing example, anti-fuse element 80 is implemented using a MOSFET inone embodiment. FIG. 3 shows a semiconductor-level view of a MOSFETanti-fuse element 80 provided over a silicon dioxide layer 85, which iscoupled to common gate transistor 90. The gate of anti-fuse element 80is connected to Vpp and after anti-fuse element 80 is programmed, itssource is created via a newly formed pinhole 83. When anti-fuse element80 is un-programmed, there is no pinhole 83 or source and the deviceoperates in the cut-off region. As noted, the anti-fuse element 80 mayalso be constructed as a capacitor structure.

Referring back to FIG. 2, anti-fuse element 80 is cascoded with commongate transistor 90 so that common gate transistor 90 may provide highvoltage protection for other elements of the memory cell 70. The gate ofcommon gate transistor 90 is consistently biased by DC voltage V_(CMN)on a common line 92 which is commonly connected to the gates of alltransistors 90 of cells 70 of the array 60. The drain of common gatetransistor 90 is connected to anti-fuse element 80 and the source isconnected to storage region 130 which may be formed in a substrate as afloating diffusion region. When anti-fuse element 80 is programmed, theresistance between Vpp and the drain of common gate transistor 90 isrelatively low. Accordingly, when a high gate voltage V_(CMN), forexample Vpp/2, is applied on the gate of common gate transistor 90, thevoltage seen by storage region 130, is equal to V_(CMN)−V_(T), whereV_(T) is a threshold voltage of common gate transistor 90. As a result,other cell elements, i.e., switch transistor 100 and amplifiertransistor 120, are protected from the high voltage stress required toprogram anti-fuse element 80.

FIG. 4 shows a three transistor embodiment of an anti-fuse cell 170. Thethickness T of gate oxide 85 shown in FIG. 3 may be decreased to lowerthe voltage Vpp required to program anti-fuse element 80. If gate oxide85 thickness T is lowered enough to allow a low programming voltage Vppof 5V or less, common gate transistor 90 is no longer needed and may beremoved, resulting in a smaller cell 170.

Referring back to FIG. 2, anti-fuse cell 70 includes storage region 130for storing charge in programmed cells. Storage region 130 is connectedto a switch transistor 100, which is connected between storage region130 and bit line BL1. Switch transistor 100 is controlled at its gate bya signal on word line WL1 to select a row which is accessed along with aselected column and associated bit line BL1 during memory cell 70programming and readout.

Storage region 130 is also connected to the gate of an in-cell amplifiertransistor 120 which provides an increased load driving capability to anoutput column line V_(OUT1), V_(OUT2), etc of a memory cell 70. The loadof one readout column in the array (e.g., V_(OUT1)) is a mainlycapacitive load which slows down the response to signal change. The timeneeded to charge up or discharge the column lines Vout1, Vout2, etc.,can be described by:T _(settle)=(C _(col) _(—) _(load) *Vsig)/I _(col) _(—) _(bus)where I_(col) _(—) _(bus) is the column line current and C_(col) _(—)_(load) is the column line capacitive load. The column line capacitiveload C_(col) _(—) _(load) usually comes from metal routing parasiticcapacitance and the thousands of junctions connected to the Vout lines.As can be seen, the settle time T_(settle) is inversely proportional tothe column line's current I_(col) _(—) _(bus). Compared to anti-fusememory cell 20 of FIG. 1, anti-fuse cell 70 provides an amplified signalwhich proportionally increases column current I_(col) _(—) _(bus). Anincreased column line current allows a faster settle time T_(settle) andaccordingly faster readout speed.

Memory cell 70 also includes a row-select transistor 110, which couplesamplifying transistor 120 to a column line, e.g., Vout1. Row selecttransistor 110 has a gate controlled by a signal on word read line WR1.

A selected cell of the array, e.g. memory cell 70, is programmed byproperly setting the voltage at BL1 and WL1 to program only memory cell70, while leaving all unselected memory cells un-programmed. Table 1shows a programming truth table for voltages on all BL, WL and WR linesin array 60 to program a selected memory cell 70.

TABLE 1 State V_(BL) V_(WL) V_(WR) SC/SR 0 Vdd 0 SC/UR 0 0 0 UC/SR VddVdd 0 UC/UR Vdd 0 0 Key: SC = Selected Column UC = Unselected Column SR= Selected Row UR = Unselected Row V_(BL) = Bit Line Voltage V_(WL) =Word Line Voltage V_(WR) = Word Read Voltage

Referring to the cells in FIG. 2 array 60 by an (X,Y) addressing schemewhere cell 70 is (1,1), according to TABLE 1, cell 70, as the selectedcell, is in state SC/SR. To select only cell 70 for programming, bitline col 1 (“BL(1)”) is set to 0, word line row 1 (“WL(1)”) is set toVdd, and word read row 1 (“WR(1)”) is set to 0. Under these conditionsthe voltage across anti-fuse element 80 is close to: Vpp−0. Therefore,anti-fuse element 80 is biased at close to the full programming voltageVpp and will be programmed. The surrounding cells, however, remainunaffected. The cell 170 in position (1,2) will receive signalsaccording to TABLE 1 corresponding to state UC/SR (selected row,unselected column). Accordingly, the voltages applied to cell 170 areBL(2) set to Vdd, WL(1) set to Vdd, and WR(1) set to 0. Under theseconditions the voltage across the anti-fuse element 175 of memory cell170 is: Vpp−Vdd. The difference results in a voltage insufficient forprogramming anti-fuse element 175, and memory cell 170 remainsun-programmed. The memory cell 180 in position (2,1) will receivesignals according to TABLE 1 corresponding to state SC/UR. The voltagesapplied to the cell are BL(1) set to 0, WL(2) set to 0, and WR(2) set to0. Under these conditions the switch transistor 200 is turned off andone end of anti-fuse element 210 is floating. The voltage acrossanti-fuse element 210 will be close to 0, accordingly it will not beprogrammed. The memory cell 190 in position (2,2) will also receiveWL(2) set to 0, resulting in switch transistor 220 being turned off aswell, with similar results to memory cell 180. All other memory cells inarray 60 would receive signals corresponding to those from one of thethree possible states of unselected cells and will therefore remainun-programmed.

FIG. 5 shows a timing diagram of a programming sequence for a selectedmemory cell, e.g., memory cell 70. The common gate voltage V_(CMN) isapplied transistor 90. The bit line BL voltage then drops low, e.g., toground. Next, the word line WL is pulsed high and remains high for atime t_(program) sufficient to allow the voltage Vpp to program theanti-fuse element 80 in the selected cell. The anti-fuse element 80 isprogrammed and word line WL returns low. The bit line BL then returnshigh.

Referring again to FIG. 2, array 60 includes column readout circuitry150 for reading data out of array 60. Data read out for a row ofanti-fuse cells 70 in an array occurs when a signal on word read lineWR1 goes high and turns on transistor 110, and signal V_(CMN) is highenough, preferably approximately Vpp/2, to at least partially turn ontransistor 90. The state of the signal on storage region 130 withtransistors 100 and 90 on will depend on whether anti-fuse element 80 isprogrammed or not. If anti-fuse element 80 is programmed, a relativelyhigh level of charge appears on storage region 130 such that a highsignal level is provided by transistor 120. If anti-fuse element 80 isnot programmed, storage region 130 has a relatively low level of chargeand a relatively low signal is provided by transistor 120. Various typesof readout circuitry used in array based memory devices may be employed.In one embodiment, readout circuitry 150 comprises a comparator (shownin FIG. 2A) at each column line, e.g. V_(OUT1), to digitize the outputsignal directly. In another embodiment, readout circuitry 150 comprisesa sense amplifier 260 (shown in FIG. 2B) each column line which issensitive to signal change. A sense amplifier 260 may be used to resolvedifferences in signal output and to quickly toggle the output inapplications in which high readout speed is critical. Alternatively, areadout amplifier 265 may be used on the column lines to amplify theoutput signal and improve readout accuracy (shown in FIG. 2D). Inanother embodiment, readout circuitry 150 comprises correlated doublesampling circuitry, including a sample and hold circuit 270,differential amplifier 280 and analog to digital converter 290, as isknown in the art of CMOS pixel array readout (shown in FIG. 2C).

FIG. 6 shows a timing diagram of a readout sequence for a selectedmemory cell 70 of a selected row of memory cells using a correlateddouble sampling readout circuitry depicted in FIG. 2C. First, word readsignal WR1 goes high to turn on row select transistor 110. Then wordline signal WL1 is pulsed high, resetting storage region 130 to a groundlevel while bit line BL1 is held at ground potential. A sample and holdreset SHR signal is pulsed to readout storage region 130 reset voltageon column lines Vout1 into a first capacitor of sample and hold circuit270. Word line signal WL1 then drops low, initiating integration of thestorage region 130 in all cells having a programmed anti-fuse element80. During integration time t_(INT), storage region 130 in cells havinga programmed anti-fuse element 80 will have a charge corresponding to ahigh voltage, Vdd on signal line Vpp. At the end of t_(INT), a sampleand hold signal SHS is pulsed to sample the memory cell 70 output signalvoltage onto column line Vout1 and into a second capacitor of sample andhold circuit 270. The SHR memory cell output values are subtracted fromthe SHS memory cell output value in differential amplifier 280 to cancelout common noise to improve the noise margin. The output signal ofprogrammed cells will be near Vdd. For un-programmed cells, theresultant output signal is near ground potential. The respectiveresultant output signals may then be digitized by an analog to digitalconverter (ADC) 290 to provide output signals representing theprogrammed state of the memory cells of the selected row.

The FIG. 2C embodiment may be useful to integrate along with pixel cellsof a CMOS imager array, where the readout circuit, including sample andhold circuit 270, differential amplifier 280, and ADC 290, can be sharedwith the readout circuit used for pixel array signal readout.

FIG. 7 is a block diagram of a processing system, for example, a camerasystem 700 utilizing a one-time programmable memory device 710constructed in accordance with an embodiment of the present invention.Although illustrated as a camera system the system 700 may also be acomputer system, a process control system, or any other system employinga processor and associated one-time programmable memory. The system 700includes a central processing unit (CPU) 720, e.g., a microprocessor,that communicates with the memory device 710 and one or more I/O devices750 over a bus 770. It must be noted that the bus 770 may be a series ofbuses and bridges commonly used in a processor system, but forconvenience purposes only, the bus 770 has been illustrated as a singlebus. The processor system 700 may also include random access memory(RAM) device 720, an imaging device 740, and some form of removablememory 760, such a flash memory card, or other removable memory as iswell known in the art.

It is again noted that the above description and drawings illustrateembodiments of the present invention. It is not intended that thepresent invention be limited to the illustrated embodiments.Modifications can be made to the embodiments. Accordingly, the inventionis not limited by the foregoing description or drawings, but is onlylimited by the appended claims.

1. A method of reading memory cells containing an anti-fuse element, thememory cells being arranged in a cell array, the method comprising:selecting a row of memory cells for readout; charging a storage regionof all programmed memory cells in the row in accordance with aprogrammed state of an anti-fuse element within each memory cell byactivating a common gate transistor between each anti-fuse element andstorage region; and reading out output signals associated with the levelof charge stored in the storage region of all memory cells in the row.2. The method of claim 1, wherein the reading out of the output signalscomprises, for each cell: sampling a signal representing a reset valueof the storage region; sampling a signal representing a value of thestorage region set by the state of the anti-fuse element; and outputtingan output signal equal to the difference between the sampled signalvalue signal and the reset value signal.
 3. The method of claim 1,wherein the output signals are digitized by respective comparators. 4.The method of claim 1, wherein the output signals are read out byrespective sense amplifiers.
 5. A memory device, comprising: a pluralityof memory cells arranged in an array of rows and columns, at least oneof the memory cells comprising: a one-time programmable anti-fuseelement; a storage region for storing a charge related to the programmedstate of the anti-fuse element, and an amplifier transistor having agate connected to the storage region for amplifying a signal from thestorage region and providing the amplified signal as an output signal; aplurality of pixel cells arranged in an array of rows and column andintegrated with the memory cell array; and a common readout circuitoperable to read output signals from the memory cell and pixel cellarrays.
 6. The memory device of claim 5 wherein the one-timeprogrammable anti-fuse element comprises a MOSFET transistor.
 7. Thememory device of claim 5 wherein the anti-fuse element comprises acapacitor.
 8. The memory device of claim 5, wherein the common readoutcircuit comprises: circuitry for reading out the at least one memorycell as a reset state of the storage region and as a signal state of thestorage region which is determined by the state of the one-timeprogrammable anti-fuse element.
 9. The memory device of claim 5, whereinthe readout circuit further comprises sample and hold circuitry forstoring a reset state and signal state of a memory cell or pixel cell,and a differential amplifier for subtracting the signals stored in thesample and hold circuitry.
 10. The memory device of claim 5, wherein thereadout circuit comprises a sense amplifier.
 11. The memory device ofclaim 5, wherein the readout circuit comprises a comparator.
 12. Thememory device of claim 5, wherein the readout circuit comprises areadout amplifier.
 13. The memory device of claim 5, wherein the memorycell and pixel cell arrays share common row and column lines.